This invention relates in general to foam-filled structures and, more specifically, to methods of making polyimide foam filled structures such as honeycomb panels.
Polyimide resins, as coatings, adhesives, foams and the like have come into widespread use due to their chemical inertness, strength, high temperature resistance and flame resistance. Polyimide foam in the form of sheets and panels are often used as thermal insulation in high temperature environments. Typical polyimide foams include those described in U.S. Pat. Nos. 4,425,441 (Gagliani et al), 4,426,463 (Gagliani et al), 4,518,717 (Long et al), 4,562,112 (Lee et al), 4,621,015 (Long et al) and 4,647,597 (Shulman et al).
Sheets and panels of foam are generally made by causing a layer of liquid or powder precursor on a flat surface to foam without restriction, then slicing the foam at a desired thickness parallel to the surface, to remove the rind that forms on the free surface. This generally produces a foam sheet of optimum uniformity and low density. These sheets may then be adhesively bonded to face sheets to form walls, insulating panels or the like. While useful in many applications, these panels may not have sufficient strength and stiffness for some applications.
The liquid or powder polyimide foam precursor may also be heated to the foaming and curing temperatures in a closed mold coated with a mold release to form a sheet or other desired shaped foam product as described, for example, by Gagliani et al in U.S. Pat. No. 4,425,441. While a variety of shapes may be formed, the restricted foam expansion often produces foams of uneven density and higher overall density than is generally desired. Foaming may also take place between face sheets so that the foam bonds to the face sheets, forming a composite panel. Again, the foam tends to be uneven in density and higher in density than is preferred for insulation and other applications. Also the final structure may not have sufficient strength and stiffness for some purposes.
As described by Ferro et al in U.S. Pat. No. 4,898,763, fully cured polyimide foam has been subjected to steam at high temperatures resulting in hydrolysis of the foam, rendering it non-flexible and non-resilient. The foam can then be pierced by pressing it into a mold such as a honeycomb core. This foam, however, has less desirable physical properties as a result of this complex, expensive and degrading process.
Thus, there is a continuing need for improved methods for producing complex, high strength, structures filled with uniform low density polyimide foam.